1. Field of the Invention
The present invention relates to a process for a separation of naphthoquinone and phthalic acid. More particularly, it relates to a process for separation for naphthoquinone and phthalic acid from a reaction mixture gas produced by a catalytic vapor phase oxidation of naphthalene by washing the reaction mixture gas with water and collecting naphthoquinone and phthalic acid with water as an aqueous slurry and extracting naphthoquinone from the aqueous slurry with an organic solvent.
2. Description of Prior Arts
It has been proposed to produce naphthoquinone by contacting a reaction mixture gas formed by a catalytic vapor phase oxidation of naphthalene with water to collect main products of naphthoquinone and phthalic anhydride as an aqueous slurry of naphthoquinone and phthalic acid and heating the aqueous slurry at 60 to 95.degree. C. to dissolve phthalic acid and extracting with an organic solvent which can be separated from water and has different specific gravity and a boiling point of 80 to 145.degree. C. and separating and concentrating and drying naphthoquinone (Japanese Unexamined Patent Publication No. 47937/1975).
When a crude naphthalene is used in an industrial operation the reaction mixture gas formed by the catalytic vapor phase oxidation contains naphthoquinone, phthalic anhydride, maleic anhydride and a small amount of sulfuric anhydride derived from an oxidation of sulfur-containing compounds of thionaphthene etc. which are included as impurities in naphthalene.
It has been also proposed to produce naphthoquinone by a catalytic vapor phase oxidation of naphthalene in the presence of sulfur or sulfur-containing compounds (Japanese Unexamined Patent Publication No. 35369/1974). The reaction mixture gas also includes sulfuric anhydride.
In the industrial operation by the former process, it is necessary to combine a process for recovering phthalic acid as the by-product. In the practical operation, the aqueous slurry of phthalic acid and naphthoquinone is heated to dissolve only phthalic acid, and naphthoquinone is extracted with a solvent and the aqueous phase is cooled to crystallize phthalic acid and the crystallized phthalic acid is separated by a filtration.
In this case, a large amount of phthalic acid remains in the filtrate. In order to recover phthalic acid in high efficiency, the filtrate is recycled whereby the by-products of sulfuric acid and maleic acid are accumulated and the operation is carried out under the condition of several % of sulfuric acid content and several % of maleic acid content.
When a crude naphthalene is used as a raw material in the processes for recovering naphthoquinone and phthalic acid as disclosed in Japanese Unexamined Patent Publication No. 47937/1975, a purity and a yield of the resulting naphthoquinone are lowered. Sometimes, the naphthoquinone having relatively low purity of about 93% is obtained.